1. Field of the Invention
This invention relates to a range detecting method and apparatus, and more particularly to a range detecting method and apparatus for electrically detecting the amount of spatial relative deviation between two images by adopting the range finding principle of the base line two image-coincidence range finder, and especially to a range detecting method and apparatus in which the two images are scanned to obtain image scan signals relating to these two images, whereby the amount of spatial relative deviation between the two images is detected from the two image scan signals.
2. Description of the Prior Art
There have heretofore been proposed various range detecting methods and apparatus using the described electric two-image coincidence detecting system, i.e., the range finder in which two images of an object having a relative positional parallax corresponding to the object distance and formed by a range finding optical system are received by photoelectric light receiving means so that the quantity of the relative positional parallax between the two images is obtained from the output of the light receiving means to thereby know the object distance, or the automatic focus adjusting device in camera adopting such a range finder.
For example, according to Japanese Patent Publication No. 5773/1973 (published Feb. 20, 1973 and entitled "Automatic Focus Adjusting Device in Camera") assigned to the assignee of the present application, there is proposed a range detecting apparatus in the form of an automatic focus-adjusting device in a camera in which a pair of photoconductive elements designed such that the resistance values thereof may vary with positional changes in the images on the light receiving surfaces thereof are juxtaposed, then the images of the same object are formed on these elements by a range finding optical system comprising a pair of image forming lenses fixedly positioned with an interval of base line length, and a difference between the resistance values of these two elements is detected by the utilization of the fact that the amount of the relative positional parallax between the formed image on the two elements corresponds to the object distance, thereby detecting the object distance.
In such a proposed apparatus, however, the photoconductive element itself is of a very peculiar construction and accordingly, when a pair of such elements are used, it is extremely difficult to coincide the response characteristics of the two elements in an ideal condition. After all, the error signal is increased to deteriorate the detection accuracy, so that even if there is a relative positional parallax between the images on the two elements, the resistance values of these two elements become identical with each other, thus causing a mistake in range detection.
To overcome such a problem, there has been proposed a range detecting apparatus or an automatic focus adjusting device in which the aforementioned two images are purely electrically scanned by the utilization of photoelectric light receiving means to thereby obtain image scan signals relating to these two images, and the amount of relative positional parallax between the two images is obtained from the two image scan signals to thereby know the object distance, or in which the information on the relative positional parallax between these two images are used to automatically accomplish the focus adjustment of the objective lens system in a camera with respect to the object.
For example, according to U.S. Pat. No. 3,898,676 assigned to the same assignee as that of the present invention (filed Dec. 20, 1975, granted to Horsoe, et al, entitled "Distance Detecting Device";, there has been proposed an automatic focus adjusting device in which an array of photosensors is employed as the photoelectric light receiving means for receiving each of the two images and these photosensor arrays are simultaneously driven to obtain time-sequentially the photoelectric output of each photosensor in these arrays, whereby the two images are purely electrically scanned simultaneously and the resulting image scan signals relating to the two images are converted into waveform signals through low-pass filters, whereafter such signals are passed to a phase discriminator which detects the phase difference between these image scan signals, and the output from the phase discriminator is used to operate a servomotor to shift an objective lens system along its optical axis while shifting one of the two images relative to the other, whereby a point at which the phase difference between the image scan signals relating to the two images becomes zero, in other words, a point at which the relative positional parallax between the two images becomes zero, is used as the in-focus position of the objective lens system with respect to the object.
Also, according to Laid-Open Japanese Patent Application No. 45556/1976 (laid-open on Apr. 19, 1976, entitled "Distance Detecting Method and Apparatus), distance detecting method and apparatus are proposed in which a self-scanning image sensor (a kind of photosensor array) is employed as the photoelectric light receiving means for receiving each of said two images and the coincidence or non-coincidence between the image scan signals relating to the two images obtained from image sensors when the two images are repetitively scanned by these image sensors is detected by a coincidence detecting circuit while the scanning start timing of one of the image sensors is varied with respect to the scanning start timing of the other image by a variable delay circuit sensor and, the amount of relative positional parallax between the two images, or the object distance, may be directly known from the amount of deviation between the scanning start timings of the two image sensors when coincidence between said image scan signals is detected by the coincidence detecting circuit.
Any of these methods and apparatuses proposed by the aforementioned U.S. Pat. No. 3,898,676 and Laid-Open Japanese Patent Application No. 45556/1976 utilizes photosensor arrays or image sensors known as photo-diode array, CCD (charge coupled device ) or BBD (bucket bridge device) to purely electrically scan two images of an object formed by a range finding optical system and utilizes the resulting image scan signals relating to said two images for the detection of the object distance or the in-focus point. Especially, the utilization of photosensor arrays or image sensors for the purely electrical scanning leads to the utilization of accurate signals properly corresponding to the patterns of the images and may thus be expected to further enhance the accuracy of the detection of the distance or the in-focus point.
Nevertheless, these methods and apparatuses according to the prior art are far from being put into practical use because they leave numerous problems still to be solved, for example, even in the processing of the image scan signals.
For example, in the apparatus proposed by the aforementioned U.S. Pat. No. 3,898,676, as already noted, the image scan signals are converted into waveform signals by being passed through low-pass filters, and then directed into a phase discriminator, which detects whether or not there is a phase difference between the two signals. However, this phase discriminator deals with analog signals, particularly waveform signals, and is therefore complicated in construction and poor in reliability and thus, particularly if the phase difference is very small, such discriminator cannot detect it accurately and this unavoidably leads to the provision of a functionally very inaccurate automatic focusing device.
Also, in the method and apparatus proposed by the aforementioned Japanese Published Patent Application No. 45556/1976, a differential amplifier or a combination of a differential amplifier and a comparator is simply employed as the circuit for detecting coincidence or incoincidence between the two image scan signals, but when considering that the signals dealt with thereby is time-series signals put out from image sensors, it is nearly impossible to detect coincidence or incoincidence between those signals solely by such a simple construction and accordingly, it can hardly be expected to accomplish highly accurate distance detection. Further, in the method and apparatus now under discussion, a variable delay circuit is used to vary the scanning start timing of one of the image sensor, whereby the amount of deviation between the scanning start timings of the two image sensors is seized as the object distance, but as is commonly recognized well, where use is made of image sensors such as charged storing type photodiode arrays or self-scanning type image sensors such as CCD or BBD, if the scanning start timing thereof, namely, the timing for imparting the start pulse, is varied, the integration time or the effective light reception time is varied to fluctuate the level of the output signal. Accordingly, in such proposed method and apparatus wherein the scanning start timing of one of the image sensors is varied with respect to the scanning start timing of the other image sensor, extreme level fluctuations are caused between the two image scan signals and therefore, even if an effort is made to compare such image scan signals to detect the coincidence therebetween, this is apparently impossible and after all, accurate distance detection is not at all possible.